Implantable devices are commonly used in medical applications. One of the more common device structures includes tubular prostheses, which may be used as vascular grafts to replace or repair damaged or diseased blood vessels. To maximize the effectiveness of such a prosthesis, it should be designed with characteristics which closely resemble that of the natural body lumen which it is repairing or replacing.
One form of a conventional tubular prosthesis specifically used for vascular grafts includes a textile tubular structure formed by weaving, knitting or braiding synthetic fibers into a tubular configuration. Tubular textile structures have the advantage of being naturally porous, which allows desired tissue ingrowth and assimilation into the body. This porosity, which allows for ingrowth of surrounding tissue, must be balanced with fluid tightness so as to minimize leakage during the initial implantation stage. Other tubular prosthesis are formed of expanded polytetrafluoroethylene (ePTFE), which may be used for smaller diameter prosthesis.
Composite ePTFE textile grafts are also known. For example, U.S. Patent Publication No. 2003/0139806 describes an inner layer of ePTFE and an outer layer of textile material bonded together by an elastomeric bonding agent to form a vascular prosthesis.
It is known to provide enhanced kink and crush resistance properties to ePTFE grafts by directly attaching a spirally wrapped polymeric coil or support element made of PTFE. The ePTFE graft is heated to a temperature sufficient to assure that the PTFE coil bonds to the graft outer surface. This method, however, does not necessarily work well for composite grafts where the material of the wrapped support structure is made from a substantially different material from the underlying graft material. For example, when the underlying graft has a textile surface, attempting to attach a wrapped PTFE support member through conventional melting methods would likely weaken the textile layer because the melting temperature of PTFE (327° C.) is substantially higher than the melting temperature range of conventional implantable textiles, such as Dacron® (polyethylenetherephthalate, 240-258° C.). This holds true for other externally wrapped support structures, which have a melting temperature range above that of the underlying graft body.
Joining of very different materials to form an integrated assembly has many challenges associated with the physical and chemical differences of the materials. For example, in some instances, a wrapped support structure is intended to be attached in a manner which prevents it from being removed without damaging the underlying graft. In other cases, a wrapped support structure must be attached in a manner sufficient to perform its kink and crush resistance role, but allow for removal without damaging the underlying graft body. Such a removable support structure would allow the physician to tailor the graft to the patient, without risk of loss of structural integrity of the overall graft structure. Differences in such inherent properties as melting temperatures, surface properties, molecular weights, biocompatibility, flexibility, solubility, as well as elongation moduli, are some of the contributing factors which make it difficult to join dissimilar polymeric structures together to make an implantable prosthesis having crush and kink resistance.
Therefore, there is a need for a composite graft that can be securably attached to a coil or other external support element without compromising the graft, and which overcomes the difficulties associated with joining dissimilar materials.